The correct answer for the two-peg test is option 'C', which states that the line of sight should be parallel to the axis of the bubble tube. Let's understand why this is the correct answer by discussing the purpose of the two-peg test and the significance of each option:

1. To make the axis of the bubble tube perpendicular to the vertical axis of the instrument:
- This option is not the correct answer because the two-peg test is not specifically performed to ensure the perpendicularity of the bubble tube axis to the vertical axis of the instrument. The purpose of the two-peg test is different.

2. To make the horizontal cross-hair lie in the plane perpendicular to the vertical axis:
- This option is also not the correct answer because aligning the horizontal cross-hair with the plane perpendicular to the vertical axis is not the objective of the two-peg test. The two-peg test serves a different purpose.

3. To make the line of sight parallel to the axis of the bubble tube:
- This option is the correct answer because it accurately describes the purpose of the two-peg test. The main objective of the two-peg test is to ensure that the line of sight is parallel to the axis of the bubble tube.

Explanation of the Two-Peg Test:

The two-peg test is a surveying technique used to check the accuracy of a leveling instrument. It is performed to verify if the line of sight of the leveling instrument is parallel to the axis of the bubble tube. The test involves the following steps:

1. Set up the leveling instrument on a tripod and level it using the foot screws.
2. Drive two pegs into the ground at a suitable distance apart, preferably within the instrument's range.
3. Take a reading on a staff or leveling rod held vertically on the first peg.
4. Rotate the instrument 180 degrees and take a reading on the staff held vertically on the second peg.
5. The difference between the two readings should ideally be zero, indicating that the line of sight is parallel to the bubble tube axis.
6. If the readings differ, an adjustment can be made to bring the line of sight parallel to the bubble tube axis by using the instrument's horizontal tangent screw.

Conclusion:

The two-peg test is performed to ensure that the line of sight of a leveling instrument is parallel to the axis of the bubble tube. This is important for accurate leveling measurements in surveying. Among the given options, option 'C' correctly describes the purpose of the two-peg test.